1. Field of the Invention
The present invention relates generally to optical pick-up apparatus used for reproducing an information signal which is recorded on a record media, and more particularly, is directed to an improved optical pick-up apparatus for causing a light beam to impinge upon a record medium through an objective lens system and for guiding a reflected light beam coming from the record medium through the objective lens system to a photodetector, so as to obtain from the photodetector an output signal representing information read from the record medium.
2. Description of the Prior Art
There has been proposed a so-called optical writable disc as one type of record disc which is available for optically writing an information signal in a record track provided thereon and optically reading an information signal written in the record tracks provided thereon. A magnetooptic disc is one of such optical writable discs and is distinguished in that erasure of information signals and writing of the information signals are able to be conducted repeatedly thereon.
The magnetooptic disc comprises a disc-shaped base, a vertically magnetized layer provided for forming a recording layer on a surface of the disc-shaped base, and a protective layer formed to cover the vertically magnetized layer. Recording of information on the magnetooptic disc is effected by erasing an information signal written therein and writing a new information signal therein, and reproduction of information from the magnetooptic disc is effected by reading an information signal written therein.
In the event of writing of information signals on the magnetooptic disc, the magnetooptic disc is rotated on an axis of rotation at a central portion thereof and a laser light beam demodulated in intensity in response to the information signals is caused to impinge upon the vertically magnetized layer of the magnetooptic disc, on which a predetermined external magnetic field acts, so as to scan the same along a plurality of convolutions of the record track formed concentrically in the vertically magnetized layer so as to surround the central portion of the magnetooptic disc, so that the vertically magnetized layer is provided with inversions in the direction of magnetization in a pattern depending upon the modulation of the laser light beam. The record track is provided with pits arranged in a predetermined pattern which are to be used for, for example, tracking servo-control.
Further, in the event of reading of information signals from the magnetooptic disc, another laser light beam having power smaller than that of the laser light beam used for writing of information signals is caused to impinge upon the vertically magnetized layer of the magnetooptic disc which is rotated so as to scan the same along the record track and a reflected laser light beam from the vertically magnetized layer is detected by a photodetector, so that a reproduced information signal corresponding to the information signal written in the magnetooptic disc, a tracking error signal for use in tracking servo-control, a focus error signal for use in focus servo-control, and other necessary signals are obtained from the photodetector. The reflected laser light beam from the vertically magnetized layer of the magnetooptic disc in the case of reading of information signals is provided, in accordance with the Kerr effect, with rotations of a polarization plane thereof in response to inversions in the direction of magnetization made in the vertically magnetized layer to represent the information signals written therein. Such rotations of the polarization plane of the reflected laser light beam are detected by means, for example, of detecting each of orthogonal polarized components, that is, so-called P-polarized and S-polarized components of the reflected laser light beam separately and comparing detection outputs of the orthogonal polarized components with each other to produce a comparison output. Then, the reproduced information signal is generated based on the comparison output. Further, the tracking error signal is produced based on a detection output of changes in the reflected laser light beam caused in response to the arrangement of pits provided on the magnetooptic disc for the tracking servo-control and the focus error signal is produced based on a detection output of positions of a beam spot formed on the photodetector by the reflected laser light beam or a detection output of the configuration of the beam spot formed on the photodetector by the reflected laser light beam.
The reproduction of information from the magnetooptic disc is carried out by, for example, a disc player which has an optical pick-up apparatus constituting an optical system for reading information from the record track formed in the vertically magnetized layer of the magnetooptic disc.
One example of a previously proposed optical pick-up apparatus for use in the disc player is schematically shown in FIG. 1 and comprises an optical system 1 formed in the shape of a single unit which contains a semiconductor laser 2, an objective lens 5, a photodetector 15 and other optical elements, and mounted on the disc player to be movable along a direction (indicated by an arrow A) of the radius of a magnetooptic disc 6 having a vertically magnetized layer in which a plurality of convolutions of a record track are concentrically formed.
In the optical pick-up apparatus shown in FIG. 1, a laser light beam emitted from the semiconductor laser 2 passes through a collimating lens 3 and is collimated thereby and enters into a polarizing beam splitter 4 as, for example, a S-polarized component, and is deflected so as to have its optical axis directed toward the objective lens 5. The laser light beam emerging from the polarizing beam splitter 4 is focused by the objective lens 5 to impinge upon the magnetooptic disc 6 so as to be provided with rotations of its polarization plane in response to inversions in the direction of magnetization which are made along the record track in the vertically magnetized layer of the magnetooptic disc 6 to represent information written in the record track and it is further modulated by the arrangement of pits disposed on the record track in the vertically magnetized layer which are to be used for the tracking servo-control. The laser light beam thus provided with the rotations of the polarization plane thereof and modulated by the arrangement of pits is reflected from the record track and is a reflected laser light beam.
The reflected laser light beam from the magnetooptic disc 6 returns through the objective lens 5 and passes through the polarizing beam splitter 4 without any change in the direction of its optical axis and enters into a beam splitter 7 which is a P-polarized component. A part of this reflected laser light beam is deflected to have its optical axis directed to a photodetector 9 through a light-receiving lens 8. The remaining part of the reflected laser light beam passes through the beam splitter 7 in a straight line to a half-wave plate 11 through a phase compensator 10.
The photodetector 9 which receives the reflected laser light beam from the beam splitter 7 generates, for example, a detection output signal obtained in response to the modulation caused on the reflected laser light beam by the arrangement of pits which is provided on the magnetooptic disc 6 for the tracking servo-control and generate another detection output signal obtained in accordance with the configuration of a beam spot formed by the reflected laser light beam on the photodetector 9, and supplies these detection output signals to an error signal generating section 12. The error signal generating section 12 produces a tracking error signal St representing deviations of a beam spot formed by the laser light beam on the vertically magnetized layer of the magnetooptic disc 6 from the center of the record track and a focus error signal Sf representing the defocussing of the laser light beam on the vertically magnetized layer of the magnetooptic disc 6, based on each of the detection output signals from the photodetector 9. The tracking error signal St and the focus error signal Sf are used respectively for the tracking servo-control and the focus servo-control in which a lens driving mechanism (not shown in the drawings) provided in relation to the objective lens 5 is caused to operate.
The reflected laser light beam having passed through the half-wave plate 11 has its polarization plane rotated by 45 degrees and enters into a Wollaston prism 13 and is separated into a pair of orthogonal polarized components, that is, the P-polarized component and the S-polarized component. The P- and S-polarized components from the Wollaston prism 13 pass through a light-receiving lens 14 to the photodetector 15. The photodetector 15 detects separately the P- and S-polarized components by two light detecting elements independently contained therein and generates two detection output signals in response to variations in the respective polarized components which are supplied to an information signal generating section 16. In the information signal generating section 16, the rotation of the polarization plane of the reflected laser light beam caused by the record track in the vertically magnetized layer of the magnetooptic disc 6 is detected by comparing the detection output signals obtained from the photodetector 15 with each other so as to produce a comparison output which varies in response to the rotation of the polarization plane of the reflected laser light beam, and then a reproduced information signal Si is generated based on the compared output. Reproduced information is derived from the reproduced information signal Si.
In the previously proposed optical pick-up apparatus described above, the Wollaston prism 13 is employed for obtaining the reproduced information signal Si, and therefore, two light detecting elements for detecting respectively the orthogonal polarized components of the reflected laser light beam are assembled in the photodetector 15. However, since the photodetector 9 for supplying the detection output signals obtained based on the reflected laser light beam to the error signal generating section 12, which contains a group of light detecting elements, is required to be provided additionally and separately from the photodetector 15, the beam splitter 7 is disposed in an optical path for the reflected laser light beam from the magnetooptic disc 6 so as to divide the reflected laser light beam into two parts, and an optical path for leading one of the divided parts of the reflected laser light beam to the photodetector 9 is provided for producing the tracking error signal St and the focus error signal Sf and another optical path is provided for leading the other of the divided parts of the reflected laser light beam to the photodetector 15 which is provided for the production of the reproduced information signal Si. This results are separately formed. This results in a large number of optical elements such as lenses and beam splitters in being required for constructing the apparatus and the number of process steps for assembling the apparatus is increased, and the cost of producing the apparatus is increased. In addition, the size of the apparatus is increased, and it is difficult to decrease the size.